In a bowl, gently combine tomatoes, gin, juniper berries, shallots, garlic, vinegar, and sea salt. Pick unbruised whole leaves (about 20) from basil to reserve as a garnish; store in a cool place. Add rest of basil, including stems, to the bowl. Let tomatoes marinate for 1 to 2 hours.

Preheat oven to 225°. Arrange bread slices on a baking sheet and brush with olive oil; add a pinch of sea salt. Dry in oven until crisp, about 20 minutes.

Adjust seasoning of tomatoes with more salt or balsamic vinegar to taste; break toasted bread into small pieces. To serve, divide tomatoes and bread evenly among 4 plates. Drizzle generously with olive oil, garnish with reserved basil leaves, and add salt and pepper to taste.